In our British Patent Specifications Nos. 2180520A and 2180521A, we have described metal can ends, having a central panel with an aperture therein and a one-piece tear-open plastics closure of a polyolefin material (e.g., polypropylene or polyethylene) moulded, e.g. by injection moulding, on to the central panel, the plastics closure comprising a plug part filling the aperture, a rim surrounding the aperture on both upper and under sides of the central panel and a pull tab joined laterally to the plug part and lying against the upper side of the central panel, the can end being provided on its under side with a coating of a similar material to that of the closure to facilitate bonding.
It would be advantageous to provide the upper side of the can end with a similar coating to protect the metal from corrosion and from the effects of double seaming the can end to a can body, but this is not practicable since the pull tab and the rim surrounding the aperture on the upper side of the can end would then bond to the can end during the moulding process and it would be impossible to lift the tab and open the closure.
It is known to apply coatings of polymers of differing characteristics to opposite sides of a metal sheet in separate sequential lamination steps, but the different thermal conditions required have previously made it impossible to achieve simultaneous lamination of different polymers having the desired characteristics for the present application.
In our copening British Patent Application No. 8724237 of 15th October 1987 we have described how simultaneous thermal lamination of films of different polymers to opposite sides of a metal sheet can be achieved by matching the softening characteristics of the different polymers.